During recent years the development of vehicle propulsion plants has been directed towards fuel economy, while maintaining a high power output to meet increasingly intense traffic. Simultaneously, environmental considerations demand a reduction of exhaust gas emissions. The standards concerning fuel economy, purification of exhaust gases and safety, the latter including capacity for acceleration and versatility, may be expected to be more rigid in the future.
In order to meet the conditions above referred to, which to some extent act against each other, a complex technique will be required. A carburetor engine (an Otto-engine) is best suited among the piston engines to meet the primary requirements concerning high load performance, low weight and small volume, as well as low production costs, but there are difficulties in obtaining a low fuel consumption, especially during part-load, which is of decided importance when private cars are concerned. This is particularly noticable when traffic safety, carrying capacity, and versatility require a comparatively high maximum power output.
It is a well known fact, that the thermal efficiency will be increased when the compression ratio is increased. In contradistinction to the diesel engine an Otto-engine is supplied with a mixture of fuel and air, and this mixture has a tendency towards an uncontrolled and detrimental ignition (so called knocking), when the compression ratio is too high. This will impose close limits upon the maximum ratio of an Otto-engine. On the other hand the power output is governed by "throttle-control" of the air inlet, which results in a lower volumetric efficiency and consequently a reduced compression ratio, and also a lower operating efficiency.
Instead of reducing the power output from all cylinders, the objective should be to reduce the number of active cylinders, and to let the still active cylinders work under a high load.
The number of occasionally operating cylinders may be reduced by locally shutting off the fuel supply to some cylinders, or by manipulating the valve gear operating mechanism. That will, however, require rather complicated control gear, and frictional losses due to the pistons reciprocating in idle position cannot be avoided.
The objective of the present invention is to provide a subdivision of the cylinders of the plant into parts, which may be switched in and out as required. It is for instance possible to form one part so it takes about 1/3 of the maximum output, and a second part suited to take about 2/3 of the maximum output. It will then be possible to operate the engine within three different output ranges, with a high efficiency in all of them, depending upon the fact that it is possible to rely upon a small throttle governing within each range, and thus a high compression ratio in the occasionally serviceable cylinders.
The improved fuel economy during part load will in the first hand result in a reduction of the exhaust gas volume, and as the combustion within the cylinders will occur about at the compression ratio for which the engine is designed, the content of dangerous emission products will be maintained at low, controlled values.